Casting method and cast article

ABSTRACT

A casting method and cast article are provided. The casting method includes providing a casting furnace, the casting furnace including a withdrawal region in a lower end, positioning a mold within the casting furnace, positioning a molten material in the mold, partially withdrawing the mold a withdrawal distance through the withdrawal region in the casting furnace, the withdrawal distance providing a partially withdrawn portion, then reinserting at least a portion of the partially withdrawn portion into the casting furnace through the withdrawal region, and then completely withdrawing the mold from the casting furnace. The reinserting partially re-melts a solidified portion within the partially withdrawn portion to fill pores formed therein with the molten material. The cast article includes a microstructure and a porosity corresponding to being formed by a process comprising partially withdrawing, reinserting, and completely withdrawing of a mold from a casting furnace to form the cast article.

FIELD OF THE INVENTION

The present invention is directed to a casting method and cast article.More specifically, the present invention is directed to a casting methodfor reducing porosity and a cast article formed therefrom.

BACKGROUND OF THE INVENTION

Various components, such as turbine buckets for gas turbine engines, areoften formed by directional solidification (DS)/single crystal (SC)casting techniques. More specifically, many components are often formedby a casting “withdrawal” technique where a melt-filled investment moldis withdrawn from a casting furnace. Withdrawing the melt-filledinvestment mold from the casting furnace permits a molten metal or alloyin the mold to cool and solidify, forming the component within the mold.

As the molten metal or alloy cools it may shrink, leading to theformation of porosity within portions of the component, such as aturbine bucket tip shroud. One method of filling the pores formed in thecomponent includes a hot isostatic pressing (hipping) process. Thehipping process is expensive.

A casting method and a cast article with improvements in the processand/or the properties of the components formed would be desirable in theart.

BRIEF DESCRIPTION OF THE INVENTION

In one exemplary embodiment, a casting method including providing acasting furnace, the casting furnace comprising a withdrawal region in alower end, positioning a mold within the casting furnace, positioning amolten material in the mold, partially withdrawing the mold a withdrawaldistance through the withdrawal region in the casting furnace, thewithdrawal distance providing a partially withdrawn portion, thenreinserting at least a portion of the partially withdrawn portion intothe casting furnace through the withdrawal region, and then completelywithdrawing the mold from the casting furnace through the withdrawalregion to produce a cast article. The reinserting partially re-melts asolidified portion within the partially withdrawn portion to fill poresformed therein with the molten material.

In another exemplary embodiment, a casting method includes providing acasting furnace, the casting furnace comprising a withdrawal region in alower end, positioning a mold within the casting furnace, positioning amolten material in the mold, partially withdrawing the mold through thewithdrawal region in the casting furnace for a withdrawal time toprovide a partially withdrawn portion, reinserting at least a portion ofthe partially withdrawn portion into the casting furnace through thewithdrawal region, and then completely withdrawing the mold from thecasting furnace through the withdrawal region to produce a cast article.The reinserting partially re-melts a solidified portion in the withdrawnportion of the mold to fill pores formed therein with the moltenmaterial.

In another exemplary embodiment, a cast article includes amicrostructure and a porosity corresponding to being formed by a processcomprising partially withdrawing, reinserting, and completelywithdrawing of a mold from a casting furnace to form the cast article.

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of a casting method according to an embodiment ofthe disclosure.

FIG. 2 shows a process view of a casting method according to anembodiment of the disclosure.

FIG. 3 shows an enhanced view of a casting method according to anembodiment of the disclosure.

FIG. 4 shows an enhanced view of a casting method including multiplereinsertions according to an embodiment of the disclosure.

Wherever possible, the same reference numbers will be used throughoutthe drawings to represent the same parts.

DETAILED DESCRIPTION OF THE INVENTION

Provided are a casting method and a cast article. Embodiments of thepresent disclosure, in comparison to casting methods and cast articlesnot using one or more of the features disclosed herein, decreaseporosity in cast articles, increase efficiency of casting article,decrease casting costs, decrease re-crystallization in cast articles,form cast articles without hot isostatic pressing (hipping), or acombination thereof.

Referring to FIGS. 1-3, a casting method 100 includes providing acasting furnace 200 (step 110), positioning a mold 210 within thecasting furnace 200 (step 120), and positioning a molten material 220 inthe mold 210 (step 130). Next, partially withdrawing (step 140) the mold210 including the molten material 220 a withdrawal distance 217 througha withdrawal region 205, for example, according to one embodiment, in alower end 201 of the casting furnace 200, provides a partially withdrawnportion 213. The molten material 220 in the partially withdrawn portion213 at least partially crystallizes to form a solidified portion 215.

After the partially withdrawing (step 140), in one embodiment, thecasting method 100 includes holding the mold 210 at the withdrawaldistance 217 for a partial withdrawal hold time, then reinserting (step150) at least a portion of the partially withdrawn portion 213 into thecasting furnace 200 through the withdrawal region 205 to form areinserted portion. The partial withdrawal hold time includes anysuitable duration, such as, but not limited to, up to about 5 minutes,between about 15 seconds and about 5 minutes, between about 30 secondsand about 5 minutes, or any combination, sub-combination, range, orsub-range thereof. The reinserting (step 150) partially re-melts thesolidified portion 215 to fill pores 214 (see FIG. 3), for example,formed in the solidified portion 215 during the partially withdrawing(step 140). The reinserting (step 150) is followed by holding thereinserted portion within the casting furnace 200 for a reinsertion holdtime, then completely withdrawing (step 160) the mold 210 from thecasting furnace 200 to crystallize the molten material 220 and form acast article 300. The reinsertion hold time includes any suitableduration, such as, but not limited to, up to about 5 minutes, betweenabout 15 seconds and about 5 minutes, between about 30 seconds and about5 minutes, or any combination, sub-combination, range, or sub-rangethereof. The partial withdrawal hold time and the reinsertion hold timeare similar, substantially similar, or different.

Referring to FIGS. 2-3, the casting furnace 200 includes any suitablecasting furnace for receiving the mold 210 (see FIG. 2), and maintaininga temperature of the molten material 220 (see FIG. 3) at or above amolten material crystallization temperature. One suitable castingfurnace includes, but is not limited to, a directional solidificationcasting furnace. In one embodiment, the casting furnace 200 receivesand/or pre-heats the mold 210 prior to positioning the molten material220 in the mold 210 (step 130). In another embodiment, the castingfurnace 200 receives and/or pre-heats a plurality of molds 210. In afurther embodiment, gaps are formed between the plurality of molds 210within the casting furnace 200. To facilitate increased control oftemperatures within the plurality of molds 210, positioning of the molds210 includes, but is not limited to, vertical indexing, calculatedguiding, thermally semi-insulating the gaps, or a combination thereof.The molten material 220 is then introduced into the mold 210 through anaperture 206 in the casting furnace 200. The aperture 206 includes anysuitable aperture, such as, but not limited to, a hole in an upper end202 of the furnace 200, a pipe, a funnel, or a combination thereof.

The mold 210 includes any suitable mold for receiving the moltenmaterial 220 and forming the cast article 300. For example, in oneembodiment, the mold 210 includes a ceramic investment shell mold havinga pour cup in communication with one or more cavities corresponding to ashape of the cast article 300. The molten material 220 includes anymaterial suitable for casting, and is selected based upon the castarticle 300 to be formed. For example, the molten material 220 forforming a turbine bucket includes any material capable of directionalsolidification and/or single crystal formation. Suitable materialsinclude, but are not limited to, metals, superalloys (for example,nickel, cobalt, or iron base superalloys), or a combination thereof. Inanother example, the molten material 220 has a composition, by weight,of about 9.8% Cr, about 7.5% Co, about 1.5% Mo, about 6% W, about 4.8%Ta, about 0.5% Nb, about 4.2% Al, about 3.6% Ti, about 0.08% C, about0.01% B, about 0.1% Hf, and a balance of Ni.

Referring to FIG. 3, the partially withdrawing (step 140) of the mold210 the withdrawal distance 217 exposes the molten material 220 in thepartially withdrawn portion 213 to reduced temperatures outside of thecasting furnace 200. The reduced temperatures outside the castingfurnace 200 cool the molten material 220 within the partially withdrawnportion 213. Cooling the molten material 220 at least partiallycrystallizes the molten material 220 to form the solidified portion 215within the partially withdrawn portion 213. The crystallization shrinksthe molten material 220, increasing a density of the crystallizedmaterial and forming the pores 214 within the solidified portion 215. Ascrystallization continues, an increase in the crystallized materialreduces or eliminates flow of the molten material 220 into the pores214. Various characteristics, such as, but not limited to, size and/orshape of the mold 210 facilitate increases or decreases in a porosity ofthe solidified portion 215.

In one embodiment, the withdrawal distance 217 is selected to correspondwith at least one area of the cast article 300 that includes increasedpore forming characteristics, such as, but not limited to, initiallywithdrawn sections, sections including increased thickness,uninterrupted sections, or a combination thereof. For example, inanother embodiment, the withdrawal distance 217 corresponds to a buckettip shroud of the turbine bucket. The withdrawal distance 217 includes,but is not limited to, between about 0.250 inches (about 0.635 cm) andabout 6 inches (about 15.24 cm), between about 0.250 inches (about 0.635cm) and about 3 inches (about 7.62 cm), between about 0.250 inches(about 0.635 cm) and about 1 inch (about 2.54 cm), between about 0.250inches (about 0.635 cm) and about 0.750 inches (about 1.910 cm), betweenabout 0.250 inches (about 0.635 cm) and about 0.500 inches (about 1.270cm), or any combination, sub-combination, range, or sub-range thereof.After the withdrawal distance 217 is reached, the partially withdrawnportion 213 is reinserted (step 150) into the casting furnace 200.

The reinserting (step 150) of the partially withdrawn portion 213partially re-melts the crystallized material formed in the solidifiedportion 215 to fill all, or substantially all, of the pores 214 with themolten material 220. The molten material 220 within the pores 214solidifies during the completely withdrawing (step 160) of the mold 210to reduce the porosity of the cast article 300. By reducing the porosityduring the partially withdrawing (step 140) and the reinserting (step150), the casting method 100 reduces the porosity of the cast article300 without post casting porosity treatments, such as, but not limitedto, hot isostatic pressing. Additionally, partially withdrawing (step140) and the reinserting (step 150) reduce the porosity of the castarticle 300 without hot isostatic pressing related re-crystallization.

During the partially withdrawing (step 140), the mold 210 is withdrawnfrom the casting furnace 200 at a partial withdrawal rate. Suitablepartial withdrawal rates include, but are not limited to, between about1 inch/hour (about 2.54 cm/hour) and about 30 inches/hour (about 76.2cm/hour), between about 1 inch/hour (about 2.54 cm/hour) and about 15inches/hour (about 38.1 cm/hour), between about 1 inch/hour (about 2.54cm/hour) and about 10 inches/hour (about 25.4 cm/hour), between about 1inch/hour (about 2.54 cm/hour) and about 7 inches/hour (about 17.8cm/hour), or any combination, sub-combination, range, or sub-rangethereof In another embodiment, the partial withdrawal rate facilitates atype and/or amount of crystallization formed in the solidified portion215. For example, during the partially withdrawing (step 140) of themold 210, decreasing the partial withdrawal rate increases a durationfor exposure of the partially withdrawn portion 213 to the reducedtemperature outside of the casting furnace 200. The increased exposureincreases the amount of crystallization formed in the solidified portion215.

During the completely withdrawing (step 150), the mold 210 is withdrawnfrom the casting furnace 200 at a complete withdrawal rate. Suitablecomplete withdrawal rates include, but are not limited to, between about1 inch/hour (about 2.54 cm/hour) and about 30 inches/hour (about 76.2cm/hour), between about 1 inch/hour (about 2.54 cm/hour) and about 15inches/hour (about 38.1 cm/hour), between about 1 inch/hour (about 2.54cm/hour) and about 10 inches/hour (about 25.4 cm/hour), between about 1inch/hour (about 2.54 cm/hour) and about 7 inches/hour (about 17.8cm/hour), or any combination, sub-combination, range, or sub-rangethereof The complete withdrawal rate facilitates a type and/or rate ofcrystallization in the cast article 300. For example, during thecompletely withdrawing (step 160) of the mold 210, the completewithdrawal rate facilitates increased amounts of either directional orprogressive solidification.

During the reinserting (step 150), at least a portion of the partiallywithdrawn portion 213 is reinserted into the casting furnace 200 at areinsertion rate proceeding in a direction opposite or substantiallyopposite a direction of the partially withdrawing (step 140). In oneembodiment, the reinsertion rate is equivalent to any of the suitablepartial or complete withdrawal rates disclosed herein. In anotherembodiment, the rate of reinsertion is greater than the suitable partialor complete withdrawal rates disclosed herein. For example, suitablereinsertion rates include, but are not limited to, between about 1inch/hour (about 2.54 cm/hour) and about 30 inches/hour (about 76.2cm/hour), between about 1 inch/hour (about 2.54 cm/hour) and about 15inches/hour (about 38.1 cm/hour), between about 1 inch/hour (about 2.54cm/hour) and about 10 inches/hour (about 25.4 cm/hour), between about 1inch/hour (about 2.54 cm/hour) and about 7 inches/hour (about 17.8cm/hour), or any combination, sub-combination, range, or sub-rangethereof.

Additionally, in one embodiment, the partial withdrawal rate, thecomplete withdrawal rate, and/or the reinsertion rate are automated byany suitable technique throughout the casting method 100. Suitabletechniques of the automation include, but are not limited to, providingtransitions between the partially withdrawing (step 140), thereinserting (step 150), and/or the completely withdrawing (step 160),automatically reversing the partially withdrawing (step 140) to providethe reinserting (step 150), automatically reversing the reinserting(step 150) to provide the completely withdrawing (step 160), varying thepartial withdrawal rate, the reinsertion rate, and/or the completewithdrawal rate, adjusting for variations in temperature within thecasting furnace 200, adjusting for the area of the cast article 300 thatincludes increased pore forming characteristics, or a combinationthereof.

In one embodiment, the partial withdrawal rate, the reinsertion rate,and/or the complete withdrawal rate, are maintained throughout each step(with the exception of an acceleration to begin each step and adeceleration to end each step). For example, the partial withdrawalrate, the reinsertion rate, and the complete withdrawal rate are about 5inches/hour (about 12.7 cm). In another embodiment, at least one of thepartial withdrawal rate, the reinsertion rate, and the completewithdrawal rate is varied throughout the casting method 100. Varyingthroughout the casting method 100 includes, but is not limited to,between steps (for example, between the partially withdrawing (step140), the reinserting (step 150), and/or the completely withdrawing(step 160)), during at least one step, corresponding to the area of thecast article 300 that includes increased pore forming characteristics,or a combination thereof. For example, in one embodiment, the partialwithdrawal rate and the complete withdrawal rate are about 3 inches/hour(about 7.6 cm/hour), while the reinsertion rate is about 10 inches/hour(about 25.4 cm/hour).

Referring to FIG. 4, in another embodiment, the casting method 100includes repeating (step 155) of the partially withdrawing (step 140)and/or the reinserting (step 150) of the mold 210. In one embodiment,during the repeating (step 155), the partially withdrawing (step 140)exposes a previously-unexposed portion 413, and the reinserting (step150) at least partially re-melts the solidified portion 215 within areinsertion segment of the previously-unexposed portion 413. Thereinsertion segment includes, but is not limited to, all, orsubstantially all, of the previously-unexposed portion 413, less thanthe entire previously-unexposed portion 413, the area of the castarticle 300 that includes increased pore forming characteristics, or acombination thereof. For example, in another embodiment, during therepeating (step 155), the partially withdrawing (step 140) exposes thepreviously-unexposed portion 413 of about 1 inch (about 2.54 cm),including the reinsertion segment of about 0.5 inches (about 1.27 cm)corresponding to an area of increased thickness. The re-melting ofsolidified portion 215 within the reinsertion segment permits the moltenmaterial 220 to fill all, or substantially all, of the pores 214 formedtherein.

In another example, the partially withdrawing (step 140) exposes thepartially withdrawn portion 213 of about 0.5 inches (about 1.27 cm), andthe reinserting (step 150) partially re-melts the solidified portion 215within the partially withdrawn portion 213. Next, the repeating (step155) of the partially withdrawing (step 140) exposes the partiallywithdrawn portion 213 of about 0.5 inches (about 1.27 cm), including thesolidified portion 215 that has been partially re-melted, as well as thepreviously-unexposed portion 413 of about 0.5 inches (about 1.27 cm).The repeating (step 155) of the reinserting (step 150) then re-melts thesolidified portion 215 formed within the previously-unexposed portion413 without re-melting the solidified portion 215 within the partiallywithdrawn portion 213 that has been partially re-melted.

In one embodiment, the casting method 100 includes partially withdrawing(step 140) the mold 210 with the molten material 220 for a withdrawaltime. The withdrawal time includes any suitable amount of timecorresponding to formation of the solidified portion 215. Suitableamounts of time include, but are not limited to, up to about 2 hours, upto about 1.5 hours, up to about 1 hour, between about 1 hour and about 2hours, up to about 0.5 hours, or any combination, sub-combination,range, or sub-range thereof. The partial withdrawal rate during thewithdrawal time includes any suitable partial withdrawal rate disclosedherein. In a further embodiment, the partial withdrawal rate during thewithdrawal time includes, but is not limited to, a constant rate, apre-set variation in the rate of withdrawal, a progressive rate, or acombination thereof.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

What is claimed is:
 1. A casting method in a casting furnace, thecasting furnace comprising a withdrawal region in a lower end, themethod comprising: positioning a mold within the casting furnace;positioning a molten material in the mold; partially withdrawing themold a withdrawal distance through the withdrawal region in the castingfurnace, the withdrawal distance providing a partially withdrawnportion, at least partially solidifying the partially withdrawn portion;then reinserting at least a portion of the partially withdrawn portioninto the casting furnace through the withdrawal region, at leastpartially re-melting the partially withdrawn portion; and thencompletely withdrawing the mold from the casting furnace through thewithdrawal region, producing a directionally solidified or singlecrystal cast article; wherein the reinserting partially re-melts asolidified portion within the partially withdrawn portion to fill poresformed therein with the molten material.
 2. The casting method of claim1, comprising varying a partial withdrawal rate during the partiallywithdrawing of the mold, the varying corresponding to portions of themold including increased pore forming characteristics.
 3. The castingmethod of claim 1, comprising varying a complete withdrawal rate duringthe completely withdrawing of the mold, the varying corresponding toportions of the mold including increased pore forming characteristics.4. The casting method of claim 1, comprising decreasing at least one ofa partial withdrawal rate and a complete withdrawal rate correspondingto a portion of the mold including increased thickness.
 5. The castingmethod of claim 1, wherein the partially withdrawing of the moldcomprises a partial withdrawal rate of between about 1 inch per hour andabout 10 inches per hour, the completely withdrawing of the moldcomprises a complete withdrawal rate of between about 1 inch per hourand about 10 inches per hour, and the reinserting comprises areinsertion rate of between about 1 inch per hour and about 20 inchesper hour.
 6. The casting method of claim 5, wherein the partialwithdrawal rate differs from the complete withdrawal rate.
 7. Thecasting method of claim 5, wherein the reinsertion rate is greater thanthe partial withdrawal rate and the complete withdrawal rate.
 8. Thecasting method of claim 5, wherein the reinsertion rate, the partialwithdrawal rate, and the complete withdrawal rate are equivalent.
 9. Thecasting method of claim 1, further comprising a partial withdrawal holdtime prior to the reinserting, and a reinsertion hold time prior to thecompletely withdrawing.
 10. The casting method of claim 1, furthercomprising positioning one or more additional molds within the castingfurnace.
 11. The casting method of claim 10, wherein the positioning ofthe one or more additional molds is selected from the group consistingof vertical indexing, calculated guiding, thermally semi-insulating gapsformed between the one or more additional molds, and combinationsthereof.
 12. The casting method of claim 1, wherein the withdrawaldistance is between about 0.250 inches and about 6.0 inches.
 13. Thecasting method of claim 1, further comprising repeating the partiallywithdrawing and the reinserting of the mold prior to the completelywithdrawing.
 14. The casting method of claim 13, wherein the repeatingof the partially withdrawing exposes a previously-unexposed portion ofthe mold, and the repeating of the reinserting partially re-melts thesolidified portion in the previously-unexposed portion of the mold. 15.The casting method of claim 1, wherein the partially withdrawing and thereinserting of the mold reduces a porosity of the cast article.
 16. Thecasting method of claim 1, wherein the partially withdrawn portioncomprises a turbine bucket tip shroud, and the cast article comprises aturbine bucket.
 17. A casting method in a casting furnace, the castingfurnace comprising a withdrawal region in a lower end, the methodcomprising: positioning a mold within the casting furnace; positioning amolten material in the mold; partially withdrawing the mold through thewithdrawal region in the casting furnace for a withdrawal time toprovide a partially withdrawn portion, at least partially solidifyingthe partially withdrawn portion; reinserting at least a portion of thepartially withdrawn portion into the casting furnace through thewithdrawal region, at least partially re-melting the partially withdrawnportion; and then completely withdrawing the mold from the castingfurnace through the withdrawal region, producing a directionallysolidified or single crystal cast article; wherein the reinsertingpartially re-melts a solidified portion in the withdrawn portion of themold to fill pores formed therein with the molten material.